As the development of the display technology of LCD, LCD has been used in various fields of display and people's requirements on the image quality of a LCD are also increasing.
Since a LCD does not emit light itself, the lighting quality of a backlight of the LCD is particularly important. Light Emitting Diode (LED) backlights have the benefits of high brightness, low power consumption and etc. as compared to conventional Cold Cathode Fluorescent Lamp (CCFL), therefore LED backlight modules have been applied in more and more LCD devices.
There are two types of LED backlight modules, which are direct-lit backlight module and edge-lit backlight module respectively. With the edge-lit backlight module, light comes in from the edges, which is different from the direct-lit backlight module, therefore the number of the LED lights can be reduced, power consumption and production cost can be lowered, and also the thickness of a display device can be smaller. In the edge-lit backlight module, a light guide plate converts parallel incident light into upward-exiting planar light. The light guide plate comprises a substrate having an incident surface for receiving a light beam, a bottom surface adjoining the incident surface, an exiting surface adjoining the incident surface and opposed to the bottom surface, and a side surface adjoining the bottom surface and the exiting surface and opposed to incident surface. In the light guide plate, light is incident on the incident surface, then reflected to the exiting surface by the bottom surface and exits. In order to further improve the luminous efficiency, mesh dots may be arranged on the bottom surface or a mesh dot layer may be attached to the bottom surface so that light can be reflected to the exiting surface more efficiently by the mesh dot and then exits. As illustrated in FIG. 1, an edge-lit backlight module comprises a light guide plate 11 on the bottom surface of which a mesh dot layer is arranged and a light source 12 that comprises LEDs. The light source 12 is located on a side of the light guide plate 11 and light from the LEDs travels through the light guide plate 11 as illustrated by the arrow in the figure.
According to Beer-Lambert law, when a parallel monochrome light beam passes through a uniform non-diffusing light absorbing substance perpendicularly, the absorbance A of the monochrome light by the light absorbing substance is proportional to the concentration c of the light absorbing substance and the thickness b of the light absorbing layer as illustrated in the following Equation 1.A=lg(1/T)=Kbc  Equation 1
wherein A is the absorbance, T is the transmittance that represents a ratio of the intensity of the transmitted light to the intensity of the incident light, K is the molar absorption coefficient, c is the concentration of the light absorbing substance, and b is the thickness of the light absorbing layer. As illustrated in FIG. 1, upon light passing through the light guide plate, the excessive absorbing of light having certain wavelength by the light absorbing substance in the substrate of the light guide plate dramatically decreases the proportion of the light having certain wavelength in the whole light spectrum. Further, the light having certain wavelength can also be absorbed by the light absorbing substance in the mesh dot layer when it is reflected by the layer. As the optical path of the light and the times that the light is reflected in the substrate of the light guide plate increase, the total absorbing of the light having certain wavelength will continue increasing, which gradually reduces the intensity of the light having certain color, causes the intensity of the light having certain color varying along the backlight illuminating direction (the backlight illuminating direction herein means the light illuminating direction parallel to the bottom surface of the light guide plate), and thus makes the chroma of the displayed image deviated. Especially when applied in large scale LCDs, edge-lit backlight modules will result in severe chroma deviation of the displayed image.